Abstract

The gut-brain axis is involved in the pathogenesis of Parkinson’s disease (PD). Although constipation is one of the identified prodromal characteristics, it is unclear whether Irritable bowel syndrome (IBS) and PD are connected or not. The objective of this study is to review the existing evidence of the relationship between IBS and PD. The systematic search of the studies published between January 2021 and January 2026 was performed in PubMed/MEDLINE, Embase, Scopus, and Web of Science following Preferred Reporting Items of Systematic Reviews and Meta-Analyses guidelines. Only the observational studies that explored the relationship between IBS-PD and adults were incorporated. Two reviewers independently carried out data extraction and risk-of-bias assessment (with the help of Risk of bias in non-randomized studies - the ROBINS-I). The study was heterogeneous, and a narrative synthesis was performed. Six studies were taken into consideration. The longitudinal studies have had some contradictory results on IBS as a risk factor of PD in some of the registry studies, but a large cohort study of 7,008 participants with Mendelian randomization did not find a cause-and-effect relationship. IBS was also quite common in established PD as compared to controls (18.1% vs. 5%) and had a higher prevalence of non-motor symptoms (NMS), notably mood disorders than motor severity. A single imaging study associated IBS in PD with increased dopamine transporter binding of the right anterior putamen. IBS is another frequent comorbidity in the case of established PD, which is associated with particular NMS and unique dopaminergic profiles. Recent findings indicate the possibility of IBS being an early clinical indicator in the PD prodrome, indicating the implication of the enteric nervous system involvement but not being an etiological risk factor.

Key words: Irritable Bowel Syndrome; Parkinson Disease; Gut-Brain Axis; Systematic Review; Prodromal Symptoms; Non-Motor Symptoms; Risk Factors